Lawn mower



27, 1940- J. D.- BOBBROFF 2,212,873

LAWN MOWER Filed June 22, 1938 Inventor:

James D. Bobbroff,

Jttorney.

Patented Aug. 27, 1940 FATE GFME

LAWN

MOWER James D. Bobbrofi, Portland, Oreg., assignor to EversharpLawnmower 00., a corporation of Nevada Application June 22, 1938, SerialNo. 215,206

4 Claims.

My invention relates to lawn-mowers of the general type wherein a rotarycutter-head makes contact with a stationary knife to form a continuouslyacting shear and more particularly to that type thereof employing flatflexible blades as typified by the subject of United States LettersPatent No. 1,818,220.

The principal object of my invention is the provision of a more nearlyuniform power requirement so that the cutter-head may be kept at a highspeed of rotation with a given power input.

A second object is the provision of durability.

Other objects and advantages of my invention will be apparent in thefollowing discourse wherein the significance of the reference charactersin the accompanying drawing, details of construction and manner of useof a typical mower embodying my invention, and the particular advantagesthereof are explained.

Figure 1 represents a view of a transverse section of the mower normalto the axis of rotation of the cutter-head.

Figure 2 represents a view of a fragment of the cutter-head partly inelevation and partly in section.

Figure 3 represents a view in perspective of a knife-carrier element.

Figure 4 represents a view of the device for retaining theknife-carriers in position, partly in radial section.

Figure 5 is a diagram representing in cylindrical projection therelative dispositions of the several cutting edges of the knives showingthe staggered and alternately inclined arrangement thereof. 1

Figure 6 represents a View of a single knif carrier therefor, and shaftin section, as the knife is flexed by engagement with an obstruction.

Figure '7 is a diagram illustrative of certain effects inherent in theoperation of ordinary mowers.

Figure 8 is a diagram illustrative of certain effects inherent in theoperation of the subject mower.

The essence of the present invention resides in the use of a relativelyheavy shaft l of hexagonal form upon which knife carriers as 2, 3, 4,and 5, of substantial weight and size are mounted by means of retainersas 6, l, 8, and 9, shown 50 in detail in Figure 4, interposed insymmetrical pairs between successive groups of carriers as 2 and 3 and 4and 5 respectively, to resolve an axial thrust imposed by a nut ll)engaged upon the shaft-threads ll, into centripetal strains 55 servingto clamp the carriers securely to the shaft surfaces, in conjunctionwith hat, flexible blades as at I2, l3, i l, and I5 secured to the saidcarriers in alternately inclined relationship, said blades being soformed at the edges thereof as to be coincident with a cylindricalsurface it of 5 a radius substantially equal to the distance between thecenter of the shaft l and the cutting edge of the stationary knife H.

In other respects the mower employs expedients of established usage inthe art. A traction wheel i8 is journaled upon the frame-casting l9 andis formed with an internal gear. engaging the customary pinion,notshown, to drive the shaft l in rotation. A roller 20 supports thestationary knife ll upon the roller-bracket 2| and a handle structure 22is pivotally mounted on the frame H3 in the usual manner. The foregoing,of course, being a typical'construction for each of two symmetrical sideunits joined by the usual tie-rod 23, a detailed description of thisconventional assembly is omitted.

The knife-carriers as 3 and i are shown in detail in Figure 5 and arecharacterized by these features: two symmetrically arranged conicalsurfaces 2t and 25, two internally disposed plane surfaces indicated at28 and El inclined to form an angle therebetween of 120 degrees, a body28 of segmentary cylindrical form, and a radially extended andlongitudinally inclined arm 29 to which the flexible, flat, knife as Mmay be attached. These carriers are of two forms differentiated byopposite and symmetrical longitudinal inclinations of the arms as 2Q toprovide for the disposition of the knife-edges in the cylindricalsurface it in staggered and alternately inclined relationshipillustrated by Figure 5.

The retainers as 6 and l are shown in detail in Figure 4 and are ofuniform pattern characterized by an internal conical surface 30 adaptedto engage the conical surfaces as 24 and 25 of the knife-carriers as 3and 4', and a plane surface of annular form indicated at 3! adapted formutual contact when arranged in symmetrical pairs as 8 and l in Figure2.

The retainers as E and i have an aperture of slightly greater diameterthan the greatest shaft 7 diameter to allow of the shifting of the,retainers radially to make contact with the three knifecarrier conesengaged thereby when slight inequalities in the axial length of theknife-carriers must be compensated for without localizing thecompressive stress of the assembly eccentrically to theshaft axis. I

The particular structure set forth above, though of great utility in theassembly and mainrotating mass.

tenance of the mower, is incidental to the chief effects accruing fromthe arrangement considered to be the gist of the present invention.

These effects are of a dynamic order and relate to the kineticproperties of the structure in operative motion.

The greater portion of the mass of the rapidly revolving cutter-head isdisposed relatively close to the axis of rotation as illustrated inFigure 8 by the circle 32 and the minor masses of the flexible bladesare disposed peripherally as denoted by the circles 33 and M inobedience to the philosophy of minimum momentum of the aggregate This isin contrast to the older structures whose properties are represented byFigure 7 where the greater masses are disposed peripherally at 35 and 36and the minor masses at 31.

The severance of the grass fibres is accomplished in this mower more bythe destructive stresses attending the impact of the knife-edges movingat high velocity in conjunction with whatever shearing effect may becontributed by the movement of the blade edge across the stationary edgein actual contact therewith or in close proximity thereto, involving thedestructive flexure of the engaged fibres, than by actual staticalshearing by the relative movement of the edges across the stationaryknife in actual contact.

Since the power requirements of a mower are definitely related to thedensity of the grasses and quantitiesthereof, an approximately uniformhigh velocity of the cutting edges must be provided for by either avariable power input or a variable engagement of the cutter-head withthe grasses, or both.

The present invention contemplates a variable engagement of thecutter-head with the grasses and the provision therefor consists in thecombination of flexible blades of a normal length adapted to wipe thestationary blade and a foreshortened length upon flexure occasioned byresistance, adapted to clear the stationary knife as illustrated inFigure 6 where the clearance is represented by the dimension Othervaluable effectsinherent in this structure are illustrated by comparisonof the Figures 7 and 8. I

In the structure represented by Figure 7, impedance of the motion of themass 35, as by contact with the stationary knife of a mower soconstructed, occasions aberrations from exact truth of rotation of theassembly represented at one extremity of the amplitude thereof by thebroken line figure. It is to be observed that the mass of the entireassembly tends to become pivotal about the point of impact representedby the point of the arrow 38,'resulting in the downward and lateraldeflection of the entire assembly represented in magnitude by thedimensions e and g in the vertical aspect and the dimension h in thelateral aspect. As well, it should be noted, the dimension frepresenting the distance between the axis of rotation and thestationary knife edge, is definitely shortened, causing an increase inthe force of contact of the moving knife with the stationary knife.Iiihe dimension d representing the angular motion of the cutter-headassembly must necessarily be of greatly diminished Value as comparedwith the next previous motion in an equal period of time.

The Figure 8 represents the characteristics of the present mower.Instead of an elongation of the radial dimension of the knife-edge uponimpact, the flexible blade allows of almost unretarded angular motion ofthe greater mass 32 and consequent shortening of the radial dimension toclear the obstruction represented by the dimension b with which thedimension 0 is complementary to equal the distance between the axis ofrotation and the stationary knife. The angular motion of the greatercentrally located, mass 32 is represented by the dimension a. It is tobe observed that the force of impact is greatly reduced and consequentlythe aberrations from exact truth of rotation of the entire assembledmass are of greatly reduced amplitudes.

It is obvious that with lesser momenta of the cutter-head, trulycylindrical form of the flexible knife-edges, more nearly uniform andhigh rotative speeds, and controlled power requirements, many advantagesaccrue which contribute definitely to the art.

Having described my invention, what I claim is:

1. The combination in a lawn-mower of a stationary knife with a coactingrotary cutter-head comprising, a hexagonal shaft, a series ofknifecarriers mounted on said shaft in groups of three each formed withsurfaces adapted to bear upon two adjacent sides of said shaft and withinclined extremities, annular retainers formed with in-* clined surfacesadapted to engage the inclined surfaces of said groups of knifecarriers, means to retain the assembly of said knife-carriers and saidretainers in compressive stress, and knives fixed upon saidknife-carriers to wipe said stationary knife.

2. The combination in a lawn-mower of a stationary knife with a coactingrotary cutter-head comprising, a relatively heavy hexagonal shaft, f

said groups of knife-carriers and said pairs of retainers in compressivestress, and knives fixed upon said knife-carriers to wipe saidstationary knife.

3. The combination in a lawn-mower of a stationary knife with. acoacting rotary cutter-head comprising, a relatively heavy hexagonalshaft, a series of knife-carriers mounted on said shaft in groups ofthree each formed with surfaces adapted to bear upon two adjacent sidesof said shaft and with inclined extremities, annular retainers formedwith inclined surfaces adapted to engage the inclined surfaces of saidknife-carriers and with plane surfaces normal to said shaft axis adaptedfor mutual engagement in symmetrical arrangement in pairs between saidgroups of knife-carriers, means to retain the assembly of said groups ofknife-carriers and said pairs of retainers in compressive stress, andflexible knives fixed upon said knife-carriers formed with the cuttingedges thereof normally disposed in a cylindrical surface passing throughthe cutting edge of said stationary-knife.

4. The combination in a lawn-mower of a stationary knife with a coactingrotary cutter-head comprising, a relatively heavy hexagonal shaft, aseries of knife-carriers mounted on said shaft in groups of three eachformed with surfaces adapted to bear upon two adjacent sides of saidshaft and with inclined extremities, annular retainers formed withinclined surfaces adapted to engage the inclined surfaces of saidknife-carriers and with plane surfaces normal to said shaft axis adaptedfor mutual engagement in symmetrical pairs between said groups of knifecarriers, means to retain the assembly of said groups of knifecarriersand said pairs of retainers therebetween in compressive stress, andrelatively light flexible knives formed with elliptical cutting edgesfixed upon said knife-carriers in a manner to provide for thedisposition of said cutting edges in a cylindrical surface in staggeredand alternately in- 5 nlined correlation.

- JAMES D.,BOBBRO-FF.

